Conventionally, when using solid state sensors, in particular semiconductors, gas detection is based on variation in the resistance of a semiconductor metal oxide such as SnO.sub.2 in the presence of a reducing gas, which variation can be related to the concentration of the gas.
However, such a method suffers from the drawback of not discriminating properly between reducing gases.
In order to improve the selectivity of such sensors, it is known to associate a catalyst with the semiconductor element constituting the sensor, the catalyst being constituted by a noble metal such as platinum or palladium, and being present at a concentration that does not exceed 1% of the weight of the oxide.
It is also known that the response of such sensors can be improved by varying the temperature to which they are raised by means of a platinum resistance element integrated in the sensor and serving to heat it.
Nevertheless, neither of those improvements to the preceding method provides a solution that is completely satisfactory, which is why suggestions have been made to use the variation in the temperature of the sensor due to the heat given off by combustion of the gas in addition to measuring conductance.
Unfortunately, implementing such a method turns out to be particularly difficult since the temperature of the sensor depends not only on the heat from the reaction, but also on variations in ambient temperature or on gas flow conditions relating to speed and direction which can give rise to temperature variations in the sensor that are of the same order of magnitude as those associated with the combustion of the gas.